Nutrient Cycling in Agroecosystems 71: 43–54, 2005. DOI 10.1007/s10705-004-5285-6
© Springer 2005
Potential of agroforestry for carbon sequestration and mitigation of greenhouse gas emissions from soils in the tropics Patrick K. Mutuo1,2, G. Cadisch1, A. Albrecht2,3,*, C.A. Palm4 and L. Verchot2 1
Department of Agricultural Sciences, Imperial College at Wye, University of London, Ashford TN 25 5AH, Kent, UK; 2World Agroforestry Centre, P.O. Box 30677, Nairobi, Kenya; 3IRD, c/o World Agroforestry Centre, Nairobi, Kenya; 4The Earth Institute, Columbia University, P.O. Box 1000, Palisades, NY 10964, USA; *Author for correspondence (tel.: 254-2-52400, fax: 254-2-524001; e-mail:
[email protected])
Key words: Agroforestry systems, Carbon sequestration, Greenhouse gas emissions, Humid tropics, Sub-humid tropics
Abstract Losses of carbon 共C兲 stocks in terrestrial ecosystems and increasing concentrations of greenhouse gases in the atmosphere are challenges that scientists and policy makers have been facing in the recent past. Intensified agricultural practices lead to a reduction in ecosystem carbon stocks, mainly due to removal of aboveground biomass as harvest and loss of carbon as CO2 through burning and/or decomposition. Evidence is emerging that agroforestry systems are promising management practices to increase aboveground and soil C stocks and reduce soil degradation, as well as to mitigate greenhouse gas emissions. In the humid tropics, the potential of agroforestry 共tree-based兲 systems to sequester C in vegetation can be over 70 Mg C ha–1, and up to 25 Mg ha–1 in the top 20 cm of soil. In degraded soils of the sub-humid tropics, improved fallow agroforestry practices have been found to increase top soil C stocks up to 1.6 Mg C ha–1 y–1 above continuous maize cropping. Soil C accretion is linked to the structural development of the soil, in particular to increasing C in water stable aggregates 共WSA兲. A review of agroforestry practices in the humid tropics showed that these systems were able to mitigate N2O and CO2 emissions from soils and increase the CH4 sink strength compared to cropping systems. The increase in N2O and CO2 emissions after addition of legume residues in improved fallow systems in the sub-humid tropics indicates the importance of using lower quality organic inputs and increasing nutrient use efficiency to derive more direct and indirect benefits from the system. In summary, these examples provide evidence of several pathways by which agroforestry systems can increase C sequestration and reduce greenhouse gas emissions.
Introduction Forest conversion and land-use change in the tropics are major factors leading to losses in carbon stocks and increasing concentration of greenhouse gases in the atmosphere. Agricultural practices lead to a reduction in ecosystem carbon stocks mainly due to removal of aboveground biomass as harvest with subsequent burning and/or decomposition, loss of soil carbon as CO2, and loss of soil C by erosion. Tropical deforestation contributes as much as 25% of the net annual CO2 emissions 共IPCC 2000兲 and 10% of the global N2O emissions 共Bouwman et al. 1995兲.
Long fallow periods that can help replenish aboveand belowground C stocks and help to protect the soil from erosion 共Wiersum 1986; Nair 1990; Brady 1996兲 are no longer sustainable as population densities increase. Thus, there is a need for developing sustainable agricultural systems to maintain and improve soil organic carbon 共SOC兲 content while mitigating land degradation and greenhouse gas emissions. One of the promising management practices to sequester aboveground and soil C, and to reduce soil degradation is adopting agroforestry and other related practices in agroecosystems 共Batjes and Sombroek 1997兲. Agroforestry has been defined as a collective
44 name for land-use systems in which woody perennials are grown in association with herbaceous plants 共crops, pastures兲 or livestock, in a spatial arrangement, a rotation, or both 共Lundgren 1982兲. The effectiveness of agroforestry systems to store C in tree biomass and soils depends on both environmental and socio-economic factors. For example, estimates have shown that the combination of woody perennials and crops has the potential to store anything between 29 and 53 Mg ha–1 aboveground C in the humid highlands of Africa, between 39 and 195 Mg ha–1 C in South America and between 12 and 228 Mg ha–1 C in southeast Asia 共Winjum et al. 1992; Dixon et al. 1993; Krankina and Schroeder 1993兲. Soil organic matter has been found to increase significantly compared to continuous cultivation in hedgerow intercropping trials using legumes 共Kang 1999兲 and in planted legume fallow in several parts of Africa 共Prinz 1986; Onim et al. 1990; Gichuru 1991; Torquebiau and Kwesiga 1996兲. However, little is known of how agroforestry practices affect the localization of C in the soil matrix and hence alters its protection against mineralization in these systems. There is a considerable body of literature on the more immediate effects of deforestation on trace gas exchange, but little is known about the contribution of different land-use systems following forest conversions to trace gas exchange 共Erickson and Keller 1997; Erickson et al. 2001兲. The majority of the studies on trace gas 共CO2, N2O and CH4兲 fluxes in the humid tropics have been conducted in natural forest systems, pastures in Latin America, paddy rice, and a few other cropping systems 共Vitousek et al. 1989; Davidson et al. 1996; Erickson and Keller 1997; Veldkamp and Keller 1997; Verchot et al. 1999; Palm et al. 2002; Baggs et al. 2002a兲. Pastures in Latin America present a complicated picture with some studies showing that they are potentially a large source of N2O 共Luizao et al. 1989兲, while others report similar or lower N2O fluxes from pastures compared to the forest 共Vitousek et al. 1989; Verchot et al. 1999兲. Studies from fertilized cropping systems in the humid tropics have shown that N2O fluxes can be as much as 10 times those of the natural systems, depending on the rates and timing of application of nitrogenous fertilizers 共Davidson et al. 1996; Erickson and Keller 1997; Veldkamp and Keller 1997; Matson et al. 1998; Baggs et al. 2002b兲. Studies of fluxes of methane from systems in the humid tropics are fewer but indicate that upland forest systems consume methane. On the other hand, ir-
rigated, or paddy rice systems, are known to be a net source of methane 共Sass 1994兲. Conversion of tropical forest soils to agriculture in general has been shown to reduce the sink strength for methane 共Keller et al. 1990; Keller and Reiners 1994; Steudler et al. 1996; Verchot et al. 2000兲. Most studies in the humid tropics have, however, focused on short term cropping systems or natural forests and hence little information is available on the effect of promising agroforestry options on CH4 dynamics. This paper analyzes results of some of the recent studies in addition to some on-going work to assess the potential of agroforestry systems for carbon sequestration and mitigating emissions of greenhouse gases and tries to elucidate possible mechanisms for the effects.
Carbon sequestration in the humid tropics Potential estimates of C sequestration and carbon losses in different land-use systems can be obtained by combining information on the aboveground, timeaveraged C stocks and the soil C values of the system. Most available data on C sequestration in managed systems in the humid tropics is in pastures, where it is widely reported that there is a possibility of increasing soil C storage after forest conversion to pastures in the Amazon, as reviewed by Lugo and Brown 共1993兲, Fisher et al. 共1994兲, Guo and Gifford 共2002兲 and Fernandes et al. 共2002兲. However, these reviews contrast with the conclusions by Cadisch et al. 共1998兲 that only well-managed pastures have potential for C sequestration. On the other hand, conversion of forest and pastures to continuous cropping systems has consistently led to declines of 50–60% in soil C stocks, as reviewed by van Noordwijk et al. 共1997兲 and Guo and Gifford 共2002兲. The C losses from converting natural forests to logged forests in Brazil, Indonesia and Cameroon ranged from 100 Mg C ha–1 to 150 Mg C ha–1 共Figure 1; Palm et al. 2000; Hairiah et al. 2001兲. These data also suggested that the majority of C was lost in the vegetation after forest conversion, with little losses from the soil organic matter pool. Further conversion to continuous cropping or pasture systems leads to loss of almost all aboveground C stocks and about 25 Mg C ha–1 from the soil organic matter pool in the surface 20 cm 共Figure 1兲. For example, soil C stocks declined by 22.5 t C ha–1 after 14 years of continuous maize cropping in the Peruvian Amazon
45
Figure 1. Aboveground and soil time-averaged carbon stocks in forests, agroforestry systems and croplands/pastures under slash-and-burn systems in the humid tropics of Brazil and Cameroon 共 , Palm et al. 2000兲, and Indonesia 共", Hairiah et al. 2001; Murdiyarso et al. 2000兲. Soil C stocks are given under the zero-line. na ⫽ not available.
共Palm et al. 2000兲. Conversion of logged forests to tree-based agroforestry systems led to losses of aboveground C ranging from 70 to 140 Mg C ha–1 and less than 10 Mg C ha–1 from the surface soil organic matter pool in sites in Sumatra, Indonesia 共Hairiah et al. 2001兲. If croplands and pastures were rehabilitated through conversion to tree-based systems, this would certainly result in net aboveground C sequestration and also in belowground C in the case of conversions from cropland. The resulting carbon sequestration would range from 10 to 70 Mg C ha–1 in the vegetation and 5 to 15 Mg C ha–1 in the topsoil, over a period of 25 years 共Murdiyarso et al. 2000; Palm et al. 2000; Hairiah et al. 2001兲. Based on the observed magnitude of changes in C stocks, the potential for rapid C sequestration in the humid tropics is mainly in the vegetation, and to a lesser extent in the topsoil, though less is known about the potential C changes in the soil at greater depths. Most analyses of changes in carbon stocks have, however, not accounted for root biomass of the trees in forests or agroforestry systems, probably because of the methodological complexity to measure them accurately. However, there is also potential for agroforestry systems to sequester some C in the root system. There is evidence that roots in agroforestry systems can have a time-averaged C stock ranging from about 6 Mg C ha–1 for shifting cultivation to about 20 Mg C ha–1 for tree
Figure 2. Time-averaged aboveground C stocks in agroforestry land use systems 共e.g., shifting cultivation, cacao and rubber agroforests, bush fallows and short improved fallows兲 in relation to their rotation ages in slash-and-burn systems in the Peruvian Amazon and Indonesia. Data was obtained from Palm et al. 共2000兲 共"兲 and Hairiah et al. 共2001兲 共⌬兲.
fallows in the top 0–50 cm soil depth 共Woomer and Palm 1998兲. If aboveground time-averaged C stocks of only the agroforestry options in slash-and-burn systems were plotted against their rotation lengths, data from Hairiah et al. 共2001兲 and Palm et al. 共2000兲 showed that the C sequestration potential in vegetation in systems was about 3.5 Mg C ha–1 y–1 共Figure 2兲, although there was a tendency of C stocks to be under the average line at rotation periods shorter than 8 years, probably because of the large contribution of crop cultivation phases in the computation of time-
46 Soil C sequestration in planted legume fallows in the sub-humid tropics
Figure 3. Development of total system 共aboveground and soil兲 C stocks in several slash-and-burn land use chronosequences 共after Woomer et al., 2000兲.
averaged C stocks. Woomer et al. 共2000兲 demonstrated that the C accumulation rates 共vegetation, soil and litter兲 for different land-use systems were not based so much on the time a system has to regrow, as on the land-use type 共Figure 3兲. Fallows established after initial cropping were able to accumulate C stocks similar to that of the original forest after about 20 years. Agroforestry systems such as bush fallows and agroforests accumulated about 60% of initial forest C stocks in about 30 years, while maintaining pasture/grassland after slash-and-burn resulted in continued gradual decline in total system C stocks. Although well-managed tropical pastures in the humid tropics have shown to maintain high soil C stocks, their effectiveness in soil C sequestration is gradually reduced as pasture production declines due to mismanagement 共de Oliveira et al. 2005兲. Additionally, Guo and Gifford 共2002兲 found that developments in C stocks after forest clearing for pasture were related to precipitation, with soil C stocks in sites receiving less than 1000 mm and those receiving more than 3000 mm rainfall declining by 5 to 15%, while the sites receiving between 1000 and 3000 mm rainfall showed an increase in soil C of 8 to 22%. Decline in soil C stocks at low precipitation after forest conversion to pasture was probably due to poor establishment of pastures.
Unlike slash-and-burn systems of the humid tropics, the more intensive continuous cropping and shortterm fallow systems in sub-humid tropics have low potential for C sequestration in vegetation because of the short rotation periods 共as illustrated in Figure 2兲. Conversion of natural vegetation for continuous agricultural production has been shown to reduce soil organic C stocks by about half in five years 共Detwiler 1986兲. Conservation tillage, mulch farming, and agroforestry have been suggested to be viable strategies to enhance soil C storage in humid and sub-humid tropical agricultural soils 共Paustian et al. 1997兲. For example, use of five- to ten-year fallows with organic amendments or low-input fertilization to subsequent crops in tropical systems on soils with low activity clays resulted in intermediate levels of soil organic matter content, between that of natural vegetation and that of continuous annual crops 共Feller 1993兲. The build-up of soil C is a function of both the quantity and quality of the biomass returned to the soil. These parameters are dependent on the tree species and the way they are combined in the different agroforestry systems. However, the quantity of litter and prunings returned to the soil is usually higher in humid and sub-humid conditions than in semi-arid conditions and is higher in fertile soils compared to poor soils. Although agroforestry has potential in semi-arid areas, inadequate rainfall can be a limiting factor for achieving high biomass production needed for the build-up of soil carbon stocks. Therefore, combined approaches are required in such systems in order to improve their potential for C sequestration, e.g., a combination of zero-tillage with cover crops and/or green manures has led to increases in C stock in the range of 0.1–0.2 Mg C ha–1 yr–1 in semi-arid zones, as reported by Lal 共2000兲. Improved fallows, as we define it here, entail the planting of one or a few tree species as a substitute to natural fallow to achieve the benefits of the latter in a shorter time 共Prinz 1986; Young 1997兲. Planted fallows have the potential to ameliorate soil fertility and increase carbon pools, although the magnitude of improvement depends on several factors including the fallow species, the length of the fallow, the density of tree planting, tree management and the soil and climatic conditions. Significant increases of SOM in the top soil have been obtained with fallows of pigeon
47 Table 1. Soil carbon contents 共g C kg–1 soil兲 in plots after planted short-term legume fallows and continuous maize cropping 共control兲 in western Kenya and SE Nigeria. W. Kenya1
W. Kenya2
SE Nigeria3
Fallow system
g C kg–1
Fallow system
g C kg–1
Fallow system
g C kg–1
S. sesban ⫹ Macroptilium atropurpureum Calliandra callothyrsus Sesbania sesban Maize control SED
12.7 12.9 13.2 11.7 0.82
Crotalaria ⫹ till Crotalaria ⫹ no-till Maize ⫹ till Maize ⫹ no-till
16.6 17.7 15.7 14.9 0.50
Cajanus cajan Tephrosia candida Maize/natural bush
22.1 20.7 18.4 1.15
Soil C contents 共0–15 cm兲 measured after 18 months of improved fallows 共Ndufa 2001兲; 2Soil C contents 共0–5 cm兲 measured after 5 fallowing phases of 6 months each, alternated with maize cropping phases in between them 共Mutuo 2003兲; 3Soil C contents 共0–5 cm兲 measured after 2 years of fallows and natural bush 共Gichuru 1991兲. 1
pea 共Cajanus cajan兲 on degraded soils in western Kenya 共Onim et al. 1990兲, Tephrosia vogelii in Cameroon 共Prinz 1986兲, Tephrosia candida and Cajanus cajan in Nigeria 共Gichuru 1991兲 and in single and mixed legume species stands in western Kenya 共Ndufa 2001兲, as shown in Table 1. A two-year fallow of Cajanus cajan in southeastern Nigeria increased soil C stocks in the top 5 cm soil depth by about 1.5 Mg C ha–1, while a 1.5-year fallow of Sesbania sesban in western Kenya increased soil C stocks in the top 15 cm depth by about 2.5 Mg C ha–1. In more recent work in western Kenya, after five fallowing phases of 6 months each with Crotalaria grahamiana, alternated with maize cropping phases of 6 months between fallow periods, soil C stocks compared to continuous maize cropping increased by between 0.9 Mg C ha–1 in fields where the fallow biomass was incorporated with tilling and about 1.6 Mg C ha–1 in fallow systems that were associated with no-till practices 共Mutuo 2003; Table 1兲. Both the improved fallows and no-till practices were important in increasing soil C stocks in the top 5 cm soil. Further results suggested that the contribution of Crotalaria fallow-derived C as determined by 13C techniques accounted for 10 and 15% in the whole soil for till and no-till practices, respectively. From this data alone, it may not be justified to deduce that tillage practices have strong negative effects on soil C sequestration, because only the top 0–5 cm depth was considered. However, it is likely that incorporation of residues may transfer organic matter to lower soil depths where it would be more protected against microbial decomposition due to the reduced aeration at the lower depths. Evidence for C sequestration is demonstrated by the stabilization of soil organic matter in terms of ‘protection’ of the different pools of organic matter
from decomposition by soil microorganisms 共van Noordwijk et al. 1997兲. Intimate chemical bonding between organics and minerals limits substrate accessibility to decomposers. Physical barriers to decomposition may result from occlusion by clay minerals and exclusion of organisms from certain pore size classes. Particulate organic materials that would otherwise be subject to rapid decomposition are bound within soil aggregates, resulting in their stabilization. Results from Mutuo 共2003兲 suggested that improved fallows contribute to soil C sequestration by C stabilization in water stable aggregates 共WSA兲. WSA obtained by wet sieving after shaking for one hour in water 共method of Gregorich et al. 共1989兲, modified by Feller et al. 1996兲 in particular are thought to offer increased protection due to absence of external pores, hence poor access by microbes to internal protected C. Increases in soil C after fallowing and no-till practices 共discussed above兲 were attributed to an increase in WSA-C especially in macroaggregates 共212–2000 m兲, whereas there were marginal increases in C associated with free organic matter 共fOM兲. Carbon in mesoaggregates 共20–212 m兲 and microaggregates 共0–20 m兲, on the other hand, tended to decline after five phases 共six months each兲 of Crotalaria fallowing 共Figure 4兲. These results therefore suggest that there was re-distribution of size classes of aggregates as a result of fallowing, leading to an increase in the C contribution of large WSA and thereby reducing the C contribution of small aggregates. Increases in size of WSA due to improved fallows further will decrease the risk of erosion in subsequent crop periods, thus contributing to the sustainability of the system. According to projections, the area of the world under agroforestry will increase substantially in the near future and this will, undoubtedly, have a great impact
48
Figure 4. The distribution of C among aggregate size-classes and free organic matter 共fOM兲 in continuous maize cropping and Crotalaria grahamiana fallow system in western Kenya 共Mutuo 2003兲.
on the flux and long-term storage of carbon in the terrestrial biosphere 共FAO 2001兲. It is essential, however, to note what lands will be converted to agroforestry. If forest lands are converted to agroforestry then there will actually be losses of carbon, though less so than if other agricultural systems were established. If, on the other hand, the vast areas of degraded lands were converted to various agroforestry systems then the carbon stored could be quite large. Calculations from the work of Schroeder 共1994兲 showed soil C storage in tropical areas of as much as 20 Mg ha–1 in sub-humid and 50 Mg ha–1 in humid tropics. However, planting trees as fallows may not always translate into the improvement of soil properties. For example, in the Naalad system of the central Philippines, where Leucaena leucocephala is used as a fallow tree, there was no significant increase in soil organic matter after 5 years of fallow 共Lasco and Suson 1999兲. The leucaena trees, however, constituted a major carbon sink.
Greenhouse gas emissions in agroforestry systems The contribution of different land-use systems in the humid tropics to the increase in atmospheric trace gases in the past has mainly focused on forests, pastures and crops. Therefore in this section, we will highlight recent studies on greenhouse gas emissions in tree-based 共agroforestry兲 systems in the humid and sub-humid tropics. Nitrous oxide Palm et al. 共2002兲 measured trace gas emissions for two years in different land-use systems established after slash-and-burn in the Peruvian Amazon. They showed that high input 共fertilized兲 cropping with tillage practices doubled N2O fluxes compared to those of low input 共fertilized兲 cropping systems 共Table 2兲. Nitrous oxide emissions from the three tree-based 共agroforestry兲 systems in that study were less than a third that of high input cropping and half that of low input cropping systems. Interestingly, N2O emissions from the tree-based systems did not vary significantly from that of the forest 共averaging about 90% that of forest兲. Data from Erickson et al. 共2001兲 indicated
49 Table 2. Average fluxes of N2O, CH4 and CO2 in cropping systems, agroforestry practices and forests in slash-and-burn systems in the Peruvian Amazon and lowland humid tropics in Sumatra, Indonesia. Land-use system
Land-use system
Cropping systems
High input cropping 31.2 Low input cropping 15.6 Cassava/Imperata 7.1 Shifting cultivation 8.6 Multistrata agroforestry 5.8 Peach palm 9.8 Jungle rubber 1.0 Rubber agroforests 12.5 – R. agroforests 共cloning兲1 Forest 9.2 Forest 5.0 Logged forest 7.2
Agroforestry systems
Forests
N2O emission 共g N m–2 h–1兲
CH4 flux 共g C m–2 h–1兲
CO2 emission 共g C m–2 h–1兲
Source
15.2 ⫺ 17.5 ⫺ 14.8 ⫺ 23.5 ⫺ 23.3 ⫺ 17.0 ⫺ 12.0 ⫺ 27.5 ⫺ 29.0 ⫺ 28.8 ⫺ 31.0 ⫺ 38.2
84.0 66.6
Palm et al. 共2002兲 Palm et al. 共2002兲 Tsuruta et al. 共2000兲 Palm et al. 共2002兲 Palm et al. 共2002兲 Palm et al. 共2002兲 Tsuruta et al. 共2000兲 Tsuruta et al. 共2000兲 Tomich et al. 共1998兲 Palm et al. 共2002兲 Tsuruta et al. 共2000兲 Tsuruta et al. 共2000兲
67.5 62.6 66.4
73.3
1
Rubber agroforestry with cloning.
that N2O ⫹ NO fluxes from secondary forests can be much higher than those observed by Palm et al. 共2002兲, especially with the presence of legumes in the stand. However, N2O fluxes from the multistrata agroforestry system and peach palm plantation reported by Palm et al. 共2002兲 were not higher than that of the shifting cultivation fallow of the same age, despite the fact that both of those systems had leguminous cover crops. Trace gas emissions from different land-use systems in the lowland humid tropics in Sumatra, Indonesia are reported in Tomich et al. 共1998兲 and Tsuruta et al. 共2000兲. Average seasonal N2O emissions in the undisturbed and logged forests were about 10 g N m–2 h–1, similar to observations by Palm et al. 共2002兲 but emissions were higher in rubber agroforests 共18 g N m–2 h–1兲 共Table 2兲, probably due to soil disturbance. In a multistorey rubber agroforest system in southern Sumatra, a system that is established through a complex succession of production stages involving the planting of crops and trees for commercial and domestic products, average annual N2O emission was 0.11 kg ha–1 y–1 共1.0 g N m–2 h–1兲 共Tsuruta et al. 2000兲. This emission rate of N2O from jungle rubber was similar to that from intact forests 共Table 2兲, but lower than that from a logged forest. The high N2O emission from logged forest was associated to disturbance. These studies showed that agroforestry systems in the humid tropics have a potential to mitigate N2O emission similar to that of forests, but higher than in continuous cropping systems, especially those receiving fertilizers. Others have noted that managed but unfertilized systems had
Table 3. Total emissions of N2O 共kg N2O-N ha–1兲 and CO2 共kg CO2-C ha–1兲 after fallow incorporation in western Kenya 共Millar 2002; Millar et al. 2004兲. N2O and CO2 were measured in the first 84 and 61 days, respectively. Data from two sites 共Dindi and Oloo farms兲 were pulled together. Total gaseous emission Treatment
kg N2O-N ha–1
kg CO2-C ha–1
Sesbania sesban Sesbania/Macroptilium Continuous maize
0.4 ⫺ 1.9 0.4 ⫺ 4.1 0.19 ⫺ 0.23
1543 ⫺ 1887 1402 ⫺ 1666 669 ⫺ 769
similar or lower fluxes than forest systems, but this follows a brief increase in flux following deforestation 共Davidson et al. 2000兲. Agroforestry systems in the sub-humid tropics are dominated by use of legume trees in farmlands, with the primary objective of maintaining or improving soil fertility. Few measurements of greenhouse gases have been done in such systems 共Baggs et al. 2001; Millar 2002兲. Data from Millar 共2002兲 show that, in improved fallow systems with leguminous trees in western Kenya, total N2O emissions in 84 days 共over a wet and dry season兲 after incorporation of Sesbania sesban, Crotalaria grahamiana and Macroptilium atropurpureum residues ranged from 0.4 to 4.1 kg N2O-N ha–1 共1.4 to 4.9 kg N2O-N ha–1 yr–1兲. These rates were higher compared to natural fallows and continuous maize cropping 共no N input兲, which averaged about 0.2 kg N2O-N ha–1 共1 kg N2O-N ha–1 yr–1兲 共Table 3兲. Assuming that the N2O emissions were completely derived from the residue N input, the proportion of added residues emitted ranged between 0.5
50 and 1.9% from improved-fallow and was about 0.2% from natural-fallow residues. The higher value compares well with the proportion 共2%兲 of fertilizer N released as N2O 共M. Keller, 2002, personal communication兲, although it has been suggested that a higher percentage of applied N may be lost in gaseous form in tropical compared to temperate agricultural systems 共Erickson and Keller 1997; Veldkamp and Keller 1997兲. This situation of legume residues behaving like N fertilizers is further evidenced by the speed at which N was released from the residues 共65–90% of the N2O emissions occurred in the first four weeks after incorporation兲. Further results from Millar et al. 共2004兲 show that N2O emissions from residues were related to their quality. They found high positive correlations between emission and N content and negative correlation with C:N ratio of the residues. In other studies, greater emissions of N2O have been reported after incorporation of residues of low C:N ratios such as those of legumes compared to those of high C:N ratios such as cereal straw 共Baggs et al. 2000兲. The polyphenol contents in residues and their ability to bind proteins in laboratory incubation have also been reported to influence 共decrease兲 N2O production 共Baggs et al. 2001兲 and it is known that many agroforestry species contain substantial amounts of such secondary plant metabolites 共Palm et al. 2001兲. Methane Current data from different countries confirm that upland primary and secondary forests are CH4 sinks 共e.g., average monthly CH4 consumption rate ⫺ 30 g C m–2 h–1兲 共Table 2兲. In contrast, agricultural systems can decrease the sink strength by 50% or more. In fact, Palm et al. 共2002兲 found that a high-input cropping system in the Peruvian Amazon produced a net flux of CH4 to the atmosphere. Agroforestry systems and low-input cropping system in the same study by Palm et al. 共2002兲 maintained a CH4 sink corresponding to approximately 60% that of the secondary forest. The data therefore strongly suggest that tree-based systems are able to partially offset CH4 emissions while conventional high-input cropping systems may exacerbate CH4 emissions. Other studies have reported similar observations of CH4 consumption in agroforestry systems less than that of forests but higher than for cropping/grassland systems 共Tomich et al. 1998; Tsuruta et al. 2000; Hairiah et al., 2001兲. For example, average methane consump-
tion by soil was highest in native and logged forests 共ranging from 28 to 38 g C m–2 h–1兲, followed by agroforestry systems 共averaging about ⫺ 22 g C m–2 h–1兲 and least in a cassava/imperata cropping system at about 15 g C m–2 h–1 in Sumatra 共Tomich et al. 1998; Tsuruta et al. 2000兲 共Table 2兲. Complementary results were obtained in upland tropical soils of Sumatra, Jambi and Lampung, where Murdiyarso et al. 共1997兲 found that the sink strength of methane was related to the intensification of different land uses. The more intensive the land was used, the weaker the methane sink strength of the soil. The observed decreases in the CH4 consumption rates in the tree-based systems and cropping systems compared to the secondary forest are likely to be associated with decreased diffusion rates due to increased soil bulk density. This was aggravated in the high-input cropping system 共data of Palm et al. 2002兲, causing net CH4 emissions where the higher soil bulk density resulted in higher water-filled pore space even at low moisture contents. The decrease in CH4 consumption was compounded by the deterioration of soil structure caused by numerous tillage operations on these soils which may result in a larger fraction of microporosity 共Davidson et al. 2000兲, hence reaching saturation at lower water content. Additionally, the CH4 emission in the high-input system could have also resulted from N fertilization because of enzymatic competition with NH⫹ 4 共Mosier et al. 1991兲. Carbon dioxide Carbon dioxide emissions were also measured in the different land-use systems in Peru 共Palm, 2002, unpublished data兲. Average monthly CO2 emissions in agroforestry systems did not vary from those of the forest and the low-input cropping system, averaging at 66.1 g C m–2 h–1. However, high-input cropping increased CO2 emissions by over 25% above the other land-use systems to 84 g C m–2 h–1 共Table 2兲. The higher CO2 emission in the high-input cropping system could be mainly attributed to the numerous 共three to four times a year兲 tillage practices, and was partly due to fertilization. Fertilization of N and P has been reported to increase microbial activity of the soil 共Fernandes et al. 2002兲. In Jambi, Sumatra, Prasetyo et al. 共2000兲 also showed that CO2 emissions increased in the two years after forest conversion to grassland. Higher emissions were also reported for rubber agroforests and secondary vegetation, suggest-
51 ing that agroforestry systems do not always reduce soil CO2 emissions. This obviously has to be put in relation to below-ground C allocation in order to assess the net C balance of these systems, as discussed above. In improved fallow systems of sub-humid western Kenya, most of the residue carbon was lost as CO2 in the first three weeks after incorporation, reaching a maximum of 70 kg CO2-C ha–1 d–1 in mixed fallow of Sesbania and Macroptilium 共Millar et al. 2004兲. Total CO2 emissions in two months after addition of residues were about double in improved fallow treatments compared to continuous maize 共Table 3兲.
Relating C sequestration to greenhouse gas emissions In an attempt to relate carbon sequestration and greenhouse gas emissions in slash-and-burn systems, the results from Palm et al. 共2000, 2002兲 and Hairiah et al. 共2001, 2002兲 were superimposed in a principal component analysis 共PCA兲. The analysis showed that there was a strong positive relationship 共accounting for 78%兲 between soil bulk density, N2O emissions and reduction in CH4 sink strength as opposed to aboveground C stocks 共Figure 5A兲. The analysis suggests that emissions of N2O and CO2, the reduction of CH4 sink strength and soil bulk densities were highest in systems with low C stocks and were lowest where aboveground C stocks were high. Emissions of CO2 and N2O and the reduction of CH4 sink strength were positively related. Similarly, a positive relationship between CO2 and N2O emissions was observed by Millar et al. 共2004兲 after legume residue additions to agricultural soils. Fertilization and tillage practices in a high-input cropping system resulting in higher emissions of CO2 and N2O are thought to be caused by increased microbial activity, while the reduction of CH4 sink strength of the soil resulted from increased water-filled pore space and soil compaction, as evidenced by the positive relation between these and increased bulk density. However, a weak relationship was observed between methane consumption and both bulk density and water-filled pore space 共Tomich et al. 1998兲, as opposed to observations by Palm et al. 共2002兲. The contradictory results suggest that a further process level analysis of causal factors of CH4 consumption/emission is probably needed to provide more robust information.
Figure 5. Results of PCA on aboveground biomass, greenhouse gases and soil bulk density: F1-F2 correlation circle 共A兲 and factor map 共B兲. On the factor map, the convex hulls bring together the three replicates of each land-use. Data from Palm et al. 共2000兲, Hairiah et al. 共2001兲 and Palm et al. 共2002兲.
Additionally, agroforestry systems and forests in the humid tropics, which have been found to have higher C stocks, did not cause higher emissions of N2O and CO2 relative to cropping systems as opposed to improved fallows in western Kenya, probably because of the poor quality 共low N and high lignin兲 of litter in tree-based systems of the humid tropics. The strong influence of residue quality on N2O and CO2 emissions is emerging clearly as an important consideration in the choice of inputs in improved fallows and other related agroforestry practices in sub-humid tropics. Thus, the weak relationship 共accounting for 16% of the variation, on the Y-axis兲 between CO2 emission and C stocks 共Figure 5A兲 indicates that soil
52 respiration is influenced by other factors such as management and root respiration apart from inputs from aboveground biomass. The factor map of the treatments in relation to aboveground C stocks and greenhouse gas emissions showed a gradient on the X-axis 共component accounting for 78% of the variation兲 from forest through agroforestry systems, low-input cropping to high-input cropping 共Figure 5B兲. This suggests that agroforestry systems tend to be intermediate land-use systems between native forests and continuous cropping systems in terms of their potential to sequester carbon, mitigate greenhouse gas emissions and maintain good soil structure. Similar trends were observed by Prasetyo et al. 共2000兲, who created a database to assess the influence of land-use/land cover changes 共LUCC兲 on aboveground carbon stocks and greenhouse gases 共N2O, CH4 and CO2兲 emission in the Pasirmayang area and the whole of Jambi Province, Sumatra. Their assessments showed that there were reductions of aboveground carbon stocks and methane absorption following LUCC from native forests. These changes were associated with increased emissions of carbon dioxide and nitrous oxide.
Conclusions In the humid tropics, agroforestry 共tree-based兲 systems can sequester C in vegetation, increasing timeaveraged C stocks in the fields up to over 60 Mg C ha–1 compared to cropping/pastures, depending on the rotation age of the land-use system. The potential for C sequestration in soil 共the top 20 cm兲 is, however, less 共25 Mg C ha–1兲 than in the vegetation. In degraded soils of the sub-humid tropics, planted legume fallows associated with no-till practices increased top soil C stocks by between 0.5 and 1.6 Mg C ha–1 y–1, which was seen to correspond with improved structural development 共aggregation兲 changes of the soil, thereby increasing the likelihood for C protection and hence, C sequestration in water-stable aggregates. Well-managed agroforestry practices in the humid tropics can mitigate N2O and CO2 emissions from soils compared to high-input cropping systems while maintaining a strong CH4 sink. The observation that N2O and CO2 emissions are increased by addition of high quality legume residues in improved fallow systems in the sub-humid tropics calls for consideration of the use of organic inputs of slower N release and increased nutrient use efficiency
of subsequent crops. However, there is likely to be lower crop yields with the use of lower quality organic inputs. Thus there is a loose balance between increasing food security in degraded areas, C sequestration and mitigation of greenhouse gases. As agroforestry options become more popular, there is an urgent need to further quantify the time dependent and spatial potentials of these systems for C sequestration and mitigation of greenhouse gases emission in varied farming units, so as to enable regional assessments.
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